1. Field of the Invention
The present invention relates to a waterborne resin emulsion and a waterborne coating, and more particularly, to a waterborne resin emulsion and a waterborne coating which are suitable for use as coatings, especially building exterior coatings and coatings for various building materials for external use in which excellent water resistance and weathering resistance are required.
2. Description of Related Art
In view of reduction of environmental impact and improvement in safety and health, building exterior coatings and coatings for various building materials for external use have been replaced at an accelerating pace by waterborne coatings. Whereby, conventional organic solvent resins, as resins for coatings used therein, have been replaced by waterborne resins.
Since building exterior coatings are used outdoors and the coating film is exposed, those having resistance to various environmental factors such as sunlight and rainwater have been required. As waterborne coatings, those having superior performances suitable for these requirements have been required. Among raw materials for waterborne coatings, a waterborne resin emulsion is most important because it exerts an influence on a superior performances.
However, coating films obtained from waterborne resin emulsions such as acrylic emulsion or coatings prepared by mixing waterborne resin emulsions with pigments had poor durability to various environmental factors.
Although various methods of enhancing durability by introducing a silicone structure into a waterborne resin emulsion have been suggested to solve this problem, the durability was not always satisfactory.
For example, Japanese Unexamined Patent Application, First Publication Hei 4-57868, Japanese Unexamined Patent Application, First Publication Hei 6-256522 and Japanese Unexamined Patent Application, First Publication Hei 8-3409 discloses those in which a silicone structure is introduced into a waterborne resin emulsion by using an alkoxysilane compound or a modifier having a silicone structure in the emulsion polymerization, while the emulsion polymerization is carried out by introducing a polymerization initiator used in the emulsion polymerization in a reaction vessel after the polymerization initiator was mixed with a monomer emulsion or dissolved in deionized water. Although a waterborne resin emulsion can be obtained by this method in the case of a water-soluble polymerization initiator, the content of silicone in particles of the resulting waterborne resin emulsion becomes nonuniform and the coating film obtained by coating with the coating containing the waterborne resin emulsion is inferior in water resistance and long-term weathering resistance. In the case of an oil-soluble polymerization initiator, the desired waterborne resin emulsion cannot be obtained because of a large numbers of blocks during the polymerization.
An object of the present invention is to provide a waterborne resin emulsion and a waterborne coating, which can form a coating film having excellent durability such as water resistance, weathering resistance or the like, by using a polymerization initiator which is soluble in an organosilicon compound having a specific structure.
To obtain a waterborne coating capable of forming a coating film having excellent durability, the present inventors have intensively researched and have found that the waterborne coating capable of forming a coating film having superior durability can be obtained by adding an emulsion of a radical polymerizable unsaturated monomer, which is obtained by emulsifying the monomer in the presence of an emulsifier, and a mixed solution of a specific non-radical polymerizable organosilicon compound and a polymerization initiator, which is soluble in the non-radical polymerizable organosilicon compound, in an aqueous medium, followed by emulsion polymerization as well as hydrolysis and condensation. Thus, the present invention has been completed.
The present invention provides a waterborne resin emulsion obtained by adding an emulsion (a) of a monomer having a radical polymerizable unsaturated bond, which is obtained by emulsifying the monomer in the presence of an emulsifier, and a mixed solution (b) of a non-radical polymerizable organosilicon compound represented by the following formula:
(R1)nxe2x88x92Sixe2x88x92(R2)4-n 
(wherein n represents an integer of 1 to 3, R1 represents an alkyl, cycloalkyl or aryl group having 1 to 16 carbon atoms, and R2 represents an alkoxy group having 1 to 8 carbon atoms or a hydroxyl group) and a polymerization initiator, which is soluble in the non-radical polymerizable organosilicon compound, in an aqueous medium; and simultaneously carrying out emulsion polymerization of the monomer having a radical polymerizable unsaturated bond and hydrolysis and condensation of the non-radical polymerizable organosilicon compound.
The present invention also provides a waterborne coating comprising a waterborne resin emulsion obtained by adding an emulsion (a) of a monomer having a radical polymerizable unsaturated bond, which is obtained by emulsifying the monomer in the presence of an emulsifier, and a mixed solution (b) of a non-radical polymerizable organosilicon compound represented by the following formula:
(R1)nxe2x88x92Sixe2x88x92(R2)4-n 
(wherein n represents an integer of 1 to 3, R1 represents an alkyl, cycloalkyl or aryl group having 1 to 16 carbon atoms, and R2 represents an alkoxy group having 1 to 8 carbon atoms or a hydroxyl group) and a polymerization initiator, which is soluble in the non-radical polymerizable organosilicon compound, in an aqueous medium; and simultaneously carrying out emulsion polymerization of the monomer having a radical polymerizable unsaturated bond and hydrolysis and condensation of the non-radical polymerizable organosilicon compound.
The waterborne resin emulsion of the present invention has superior durability and weathering resistance. Therefore, the waterborne resin emulsion of the present invention is suitable for use as resins for especially building exterior coatings and coatings for various building materials for external use in which high water resistance and weathering resistance are required. The waterborne coating of the present invention is suitable for use as building exterior coatings and coatings for various building materials for exterior use.